Carbon nanotubes are a tubular material in which a single- or multi-walled graphene sheet formed of carbon atoms is rolled into a cylinder. These carbon nanotubes have different properties, depending on the way the graphene sheet is rolled into a tube, and on the shape of the carbon nanotubes themselves, such as the diameter and crystallinity of the tube, and others, and are expected as a material that is highly attractive in characteristics of the material itself, such as electrical and mechanical properties, and specific gravity, in comparison with metallic materials.
In order to make effective use of mechanical and electrical properties of carbon nanotubes in producing composite materials in combination with other materials, it is necessary to mix carbon nanotubes uniformly into other materials. For example, when a solid material and a liquid material having carbon nanotubes dispersed therein are mixed, it is necessary to employ a liquid material in which the carbon nanotubes are individually and separately dispersed (which is referred to as a “dispersion”). However, carbon nanotubes have the property of attracting each other by van der Waals forces, and are known to form bundles or agglomerates with one another.
As methods for producing carbon nanotubes, there are known vapor-phase synthesis methods using thermal CVD in which a catalyst and a raw-material gas are simultaneously placed into a reactor to synthesize carbon nanotubes, and on-substrate synthesis methods in which a raw material gas is applied onto a substrate coated with a catalyst.
Among these methods, on-substrate synthesis methods allow the formation of carbon nanotubes in which the carbon nanotubes assemble each other and are vertically oriented, by applying a catalyst to a very smooth surface of a substrate, such as silicon and silicon oxide in particular, and growing the carbon nanotubes on the surface at a high density.
Carbon nanotubes which are produced by an on-substrate synthesis method are uniform in their diameter, layer number, and length, and exhibit high crystallinity, and therefore it is expected that carbon nanotubes having a very small number of defects can be obtained. In addition, carbon nanotubes which are produced by an on-substrate synthesis method have longer lengths and exhibit higher crystallinity, in comparison with those which are produced by a vapor-phase synthesis method, and thus have an advantage that when a composite material with other materials is formed, it is easy to obtain various properties, such as improvements in the electrical conductivity, thermal conductivity, and mechanical strength, and a suppression of the linear expansion of the composite material.
In particular, composite resins of carbon nanotubes and resins are used in various applications in many fields such as electronic parts and automobile parts. These composite resins require the properties including electrical conductivity in order to achieve antistatic performance and high thermal conductivity in order to avoid thermal expansion in a molding process and a cutting process. In the past, there have been supposed resin composite materials in which to resins are added spherical carbon materials or carbon fibers such as carbon black and Ketjen Black, or fibrous carbon materials such as carbon nanotubes, as a filler used in imparting electrical or thermal conductivity to resin moldings (See, for example, Patent Literatures 1 to 3).
As methods for kneading carbon nanotubes and a resin, methods are known in which kneading is usually carried out under high shearing force using a mill or the like. In these methods, however, a high shear operation will be required in order to achieve homogeneous mixing of a fibrous carbon material, such as carbon nanotubes, into a resin using a mill or the like.
Such methods present a problem of reducing performance in electrical conductivity and others because in the resin, the carbon nanotubes are subjected to shearing force, so that they are broken into short pieces, resulting in a decreased number of contacts with one another in the resin. Further, such methods cause the carbon nanotubes to separate away from each other in the resin, leading to the destruction of nanonet structures in which the carbon nanotubes are entangled with one another in the resin, and thus do not allow the strength, an intrinsic property of carbon nanotube, to be exhibited in the composite resin, and as a result, present a problem that the strength of the composite resin is not increased.
When short carbon nanotubes are dispersed in isolation in a resin, binding at the interface between the resin and the carbon nanotube is weak, and there is presented a problem that a phenomenon is observed in which the carbon nanotubes are released easily out of the resin.
Since carbon materials such as carbon nanotubes are expensive, on the other hand, there is a desire to reduce material costs. Therefore, it is required that carbon nanotubes be homogeneously dispersed to obtain high electrical and thermal conductivities at concentrations as low as possible. For carbon nanotubes which are to be added, it is also known that the narrower the diameters and the longer the lengths of the carbon nanotubes, the lower concentrations at which they are added provide high degrees of electrical conductivity, thermal conductivity, strength, and others.
Incidentally, Patent Literature 4 discloses a method for the modification of the surface of a resin molded article with carbon nanotubes by immersing the resin molded article in a dispersion of the carbon nanotubes, followed by treatment under an atmosphere of carbon dioxide in a subcritical or supercritical state.
However, the method disclosed in Patent Literature 4 presents a problem that the modification of a resin molded article with carbon nanotubes is affected by gravity because in this method, the carbon nanotubes are poorly dispersive, resulting in carbon nanotube agglomeration, sedimentation, or the like during the treatment under an carbon dioxide atmosphere. In addition, there is presented a problem that the resin surface of a resin molded article is swollen under an atmosphere of carbon dioxide in a subcritical or supercritical state, whereby its shape and color are affected and additional processing of the resin surface is required. Here, the “modification” refers to a state where carbon nanotubes have been attached to or fixed on the resin surface.
In addition, Patent Literatures 5 and 6 each disclose a process in which the surface of resin particles is impregnated with supercritical carbon dioxide, thereby to soften the resin surface and an ultrasonic vibration method is used to disperse and fix the carbon nanotubes in a dispersion uniformly on the resin particle surface. For stirring with ultrasonic vibration, a high-pressure generator itself is required to be placed under high-pressure carbon dioxide, resulting in a problem of increasing the cost. Transmitting of ultrasonic vibration from the outside of the high pressure vessel poses a problem that its transmitting to a fluid inside the vessel is not necessarily adequate. Thus, there is a problem that the cost is increased, in order to achieve a device capable of sufficient stirring.